In the manufacture of gas fired hot air furnaces, it is known to fabricate the furnace heat exchangers by forming opposed dish shaped or clamshell sheet metal members which are joined face-to-face to form an enclosure or combustion chamber. Conventional practice in the art of heat exchanger fabrication has been characterized by welding the perimeter seam between the two clamshell sections to form an integral heat exchanger part and by assembling one or more of the heat exchanger parts to an additional structure such as a partition plate having openings corresponding to the burner and flue gas openings in the heat exchanger. Welding processes are disadvantageous in that they are expensive, time consuming and create several environmental problems which are hazardous to production personnel and contribute to the overall expense of manufacturing heat exchangers.
One improvement in the art of manufacturing the subject type of heat exchanger is described in U.S. Pat. No. 3,940,837 to John M. Wiese and assigned to the assignee of the present invention. In the Wiese patent, the heat exchanger is formed with a peripheral flange on the opposed clamshell sections which is inserted through an opening in the partition or support plate and is expanded or folded over against the surface of the plate adjacent the opening to secure the heat exchanger to the plate. A layer of insulation material is also preferably inserted between the plate and the heat exchanger flange. However, one drawback to the arrangement in the Wiese patent pertains to the tendency for the seams formed in the flange at the juncture between the opposed clamshell sections to split or open up during the flange deforming or folding process. This, of course, is detrimental to the structural integrity of the joint formed between the parts and provides a point of leakage of combustion gases from the heat exchanger.
Accordingly, there has been a need to improve on the configuration of the joint formed between a heat exchanger of the general type described herein and a supporting plate member. The high volume production requirements for such parts and the configuration of the parts themselves has also led to a need for an improved process of assembling a clamshell type heat exchanger to a partition or support plate at the respective openings for the burner assembly and flue gas discharge. Moreover, the requirement to develop a suitable pressed joint heat exchanger and plate assembly has been accentuated by the successful development of the perimeter flange pressed joint disclosed and claimed in U.S. Pat. No. 4,298,061, also assigned to the assignee of this invention.